Aluminum has been traditionally obtained from bauxites by the Bayer process, now highly developed particularly for low silica containing ores. More recently other leaching processes involving acids have been studied as routes to alumina.
Since World War II interest has been building in a chloride route to aluminum to take advantage of savings in electricity and electrode costs incidental to the use of a chloride reduction cell, and this interest has spawned a number of process concepts to convert bauxites to aluminum chloride. These processes include chlorination of Bayer process aluminas with carbon monoxide and chlorine, carbon and chlorine (where the carbon has been intimately deposited on the alumina surface) by, for example, H. P. Alder et al, Metallurgical Society of AIME, TMS Paper Selection, LM-79-16. and A. Lansberg, Metallurgical transactions, Vol. GB, P. 207, June, 1965 and carbon and chlorine chlorinations where carbon or coke are separate solid phases (see for example U.S. Pat. No. 4,124,682). The use of the more expensive alumina is to avoid later purification of aluminum chloride which would be contaminated with iron, silicon and titanium chlorides were directly chlorinated bauxite the aluminum source.
More recently, direct chlorination of bauxites has been considered using either carbon monoxide or carbon as a reducing agent for simultaneous chlorination and reduction of the oxides of the bauxites. It has been observed by many that a preferential chlorination of iron, silicon and titanium oxides can be carried out although not to the extent that the bauxite is beneficiated to an alumina pure enough to be suitable for direct cell addition or pure enough to allow chlorination to as pure an aluminum chloride as could be made by Bayer alumina chlorination.
One of the necessities for an economic chloride aluminum extraction process is the recovery of chlorine values from the impurity chlorides produced from bauxite chlorination. Although both silicon and titanium tetrachlorides have adequate thermodynamic potential for the oxidation of these chlorides at any temperature, unfortunately this is not the case with ferric chloride. Consequently processes which make ferric chloride, such as ilmenite beneficiation or titanium tetrachloride production from ilmenites have long been faced with the problem of chlorine recovery and to date have not successfully demonstrated such recovery on a commercial scale.
Among the difficulties of the ferric chloride oxidation are the maintenance of heat balance and the establishment of adequate oxidation reaction rates within the bounds of thermodynamic constraints on the ferric chloride oxidation reaction equilibrium. Where the ferric chloride is collected as a solid, it is difficult to maintain reactor temperatures high enough to get adequate reaction rates without adding heat, a particularly difficult task in such a corrosive system. Direct addition of heat across a boundary invites corrosion of the heat transfer surface while heat generation by combustion in the reactor results in significant dilution of the chlorine formed.